Question 631373
The easiest way is to think of one train as standing still
and the other one traveling at the sum of their speeds.
When the one that is moving covers 673 miles, they collide.
{{{ 673 = ( 100 + 87 )*t }}}
{{{ 673 = 187t }}}
{{{ t = 3.599 }}} hrs
{{{ .599*60 = 36 }}} min
They will collide in 3 hrs and 36 min